readline: pause stdin before turning off terminal raw mode

On windows, libuv will immediately make a `ReadConsole` call (in the
thread pool) when a 'flowing' `uv_tty_t` handle is switched to
line-buffered mode. That causes an immediate issue for some users,
since libuv can't cancel the `ReadConsole` operation on Windows 8 /
Server 2012 and up if the program switches back to raw mode later.

But even if this will be fixed in libuv at some point, it's better to
avoid the overhead of starting work in the thread pool and immediately
cancelling it afther that.

See also f34f1e3, where the same change is made for the opposite
flow, e.g. move `resume()` after `_setRawMode(true)`.

Fixes #5927

This is a backport of dfb0461 (see #5930) to the v0.8 branch.